The present invention relates generally to power transmission units for motor vehicles, and more particularly to a power transmission unit of the type which comprises a housing secured to a cylinder block of an engine and having upper and lower chambers therein, a clutch assembly arranged within the upper chamber of the housing, a change-speed gearing arranged within the lower chamber of the housing and having a mainshaft and a countershaft journalled in parallel with the clutch output member, and an idler shaft arranged between the clutch output member and the mainshaft and journalled in parallel with the clutch output member, and in which an idler gear on the idler shaft is meshed with a driving gear on the clutch output member and with an input gear on the mainshaft, and a reverse gear on the mainshaft is able to mesh with a reverse driving gear on the idler shaft by way of a reverse idler gear on a reverse idler shaft.
In a power transmission unit of this kind, the idler shaft is supported by a pair of axially spaced bearings, and the reverse driving gear is axially spaced from the idler gear. In use of the transmission unit, a predetermined amount of lubrication oil is stored within the lower chamber of the housing to lubricate the transmission mechanism. When the transmission unit is operated, the lubricating oil is stirred up by rotation of the change-speed gearing and pumped up by rotation of the idler gear toward the driving gear on the clutch output member. Thus, the lubricating oil serves to lubricate the bearings for the clutch output member, the idler shaft, the mainshaft and the countershaft. It is, however, to be observed that the lubricating oil cannot be pumped up toward the reverse driving gear during forward drive of the transmission unit because of disengagement of the reverse idler gear from the reverse gear on the mainshaft. This means that while the bearing for the idler shaft in the region of the reverse driving gear is temporarily lubricated by the lubricating oil during reverse drive of the transmission unit, the lubricating oil does not serve to lubricate the bearing for the idler shaft during forward drive of the transmission unit.